THE ROLE OF THE CEREBELLUM IN VISUAL LEARNING: A major puzzle in understanding the mechanisms of visual behavior is to answer the question of how the brain learns to associate an arbitrary stimulus with an action. There is nothing about a red traffic light that, in itself, demands that a driver slow down and stop the car ? but everyone who drives has learned that association. The cerebellum is a part of the brain that is generally thought to control motor performance. There is a huge amount of cerebellar physiology in animal models from mouse to monkey, studying well- controlled examples of motor performance, for example the change in gain of the vestibuloocular reflex in response to muscular weakening. The cerebellum is also important in classical conditioning of the eye blink, where the unconditioned stimulus is an air puff, and the conditioned stimulus a tone. As expected, the cerebellum is connected to parts of the cerebral cortex involved with movement. However, it is also connected to parts of the cerebral cortex associated with visual perception and cognitive processing. There is a rapidly developing amount of evidence that the cerebellum has a role in cognitive processes as well as motor processes, mostly from clinical correlations, but there is no physiological evidence for cerebellar function in cognitive processes beyond classical conditioning of the eye blink reflex. In our preliminary results we have shown that the cerebellum has a signal that may be important in the visual and cognitive process of visuomotor association, rather than in the shaping of a movement. We trained Rhesus monkeys on a visuomotor association task, in which the monkey learns to associate an arbitrary symbol with a simple movement. The task begins with the monkey?s putting each hand on a different dowel, which brings a fixation point on a screen in front of the monkey, which the monkey looks at. A second later one of two symbols appears replaces the fixation for 200ms. One pattern is associated with a left hand movement and the other with a right hand movement. The monkey?s job is to figure out which symbol is associated with which hand. It usually takes between 20 and 40 trials to solve the problem. We have discovered that the activity of Purkinje cells in the cerebellar cortex tracks the monkey?s process of learning. The movement does not change as the monkey learns, and it is unlikely that the cells we have discovered are controlling the motor aspects of the task. Most cells respond to movements of both hands. Finally the signal is similar when the monkey reports the association left or right using one of two very different movements ? releasing the grasp on the dowel or lifting the hand from a plate. This R21 grant is to firm up our very preliminary results that the cerebellum is involved in visuomotor association as opposed to pure motor control. These results may change the conventional wisdom about the cerebellum, and open a new way of thinking about visual learning.